Chromium reduction



Patented July 2, f 'o UNITED STATESV PATENT OFFICE This inventionrelates to articial chromite materials, and it comprises as a newmanufac ture a granular completely oxidized ferrochromium free orsubstantially free of carbon and unoxidized metal and useful in theproduction of low carbon chromium alloys. said granular materialusuallyu containing a base such as lime, together with ferric oxide,chromium rsesquioxide vand chromium trioxide in some vform of chemicalcombination, and being available for various pur-- poses, andparticularly for the direct production of molten lo'w carbon chromiumalloys by exothermic reaction with admixed carbon-free reducing agents(silicon, metal silicides or silicon alloys, including silicon-chromiumalloys, aluminum and alloys thereof, etc); it. further comprises anadmixture of said material with one of said reducing agents in suchproportions and so admixed as to enable, initiate and develop anexothermic action with production of metal containing chromium in aheated state; it further comprises a method of making such a materialwherein a chromite ore, which may be either a natural or a beneiiciatedore, and Vadvantageously a low grade ore, is totaly reduced with anampie amount of 'carbon in an electric furnace, non-metallic impuritiesbeing slagged voil?, the

ferrochromium so produced is lne ground to a powdered material and thisne ground metallic material is completely oxidized by roasting to freeit of carbon, complete oxidation being usually accelerated bythe-presenceof lime and a little soda in admixture and roasting beingoften carried `far enough to give a substantial content of CrOa in theroasted material; and it further comprises an advantageous. method ofmaking low carbon chromium alloys in a molten .condition wherein such aroasted material is intimately admixed with a reducing agent to form 'amixed body and the body is red to produce molten metal; all as morefully hereinafter'set forth and as claimed.l

By total reduction of any chromite ore in an electric furnace, slaggingoff the gangue and using plenty of carbon, a brittle metallic productcan be obtained carrying all Ithe iron and most of the chromium. It maybe regarded as a concentrate. It is, however, high in carbon andtherefore unsuited for direct usein. chromium metallurgy where for themost part low carbon is required. Nor can it readily be roasted alone toreconstitute the original metal oxides and get rid of the carbon in asingle roasting operation. In any given granule oxidation goes inwardfrom the surface and stops while there is still unoxidized metal;theroasted product is magnetic.

' But by double roasting, that is roasting once, regrinding andreroasting, oxidation can be made complete; the product is composedWholly of oxides and carries no carbon. Nor is it magnetic. The sameresult can .be attained in a single roasting by adding bases: lime orlime and soda. Such a double roasting or roasting in the presence of abase accelerating oxidation is con` templated in the present invention.

Most of Ithe stainless Vsteel and rustless iron made today are producedby utilizing low carbon ferrochromium containing' 70 percent or' morechromium 'as a source material for supplying chromium to a bath ofmolten iron. In most cases the il'nal alloy products are required to beextremely low in carbon, content and the ratio of carbon to chromium islow in this ferrochromium.

Not only is lowcarbon ferrochromium richvin chromium an expensivematerial, butit has an .linconveniently high melting point and requirestemperatureshigher thanordinary steel melting temperatures in makingalloys. .For this reason,

t the commercial llnished chrome-iron and chrome steel alloys are forthe most part made in the electric furnace.

While the direct production of chrome yirons andchrome steels fromchromite ore is practicable and in use, the employment of low` carbonferrochromium is more favored in the art. The direct use otl chromiteores is attended with the disadvantage of a large slag volume; the slagbeing highly refractory because ofthe refractory nature o f the oregangue.

In making commercial ferrochromium, it" is usually considered necessaryto utilize onlyijqhe highest grade chromite ores; those which give uponreduction ferrochromium of a standard '70 per cent chromium content; aratio of chromium to iron 2.3 to one or better. Thus the chromite oredeposits of the world have been high graded in order to obtainferrochromium of high chro- `mium to iron ratios for making steel andiron alloys, which are mostly of a low chromium high iron ratio. Thesenished alloys may carry chromium in an amount ranging from a fraction ofone per cent to upward of, perhaps, 30 per cent as a maximum.

1 It is a general object achieved in the invention to obviate the noteddifficulties by providing a new source material in making chromiumalloys; this material being free of carbon and of gangue andrepresenting thechromium and present o iron of a chromite ore in are-oxidized condition. M,

It may be here called an artificial or synthetic chromite ore, althoughit does not have the constitution of the mineral chromite; a spinelcombination of FeO and CrnOs. The articial chromite of the presentinvention, which may advantageously be made from low grade, or highiron, chromite ores, is useful for various purposes and particularly inthe production of molten chromium alloys by exothermic reaction withadmixed silicon, etc. The reduced metal formed in small globules by theexothermic action may be carbon-free and is advantageous for alloyingpurposes in making commercial chromium alloys in which absence of carbonis desirable.

According to the present invention, a natural chromite ore, which mayadvantageously contain chromium and iron in a ratio of 1:1 or even less,is completely reduced with an ample amount of carbon in an electricfurnace, non-metallic and gangue impurities being slagged off andeliminated. The high carbon ferrochromlum metal thus produced is groundto a line powder and roasted to completely reoxidize the chromium andiron of the metal and'to eliminate carbon. Complete oxidation of groundferrochromiuin in one operation is, as stated, diiiicult but by roastingin the presence of lime and a little soda complete oxidation maybeobtained. In using lime there are, in any event, several advantages. Insubsequent reaction with silica or ferrosilicon lime is available forslagging the produced silica and accelerating its production;Ordinarily, in roasting in the presence of admixed lime I use aboutequal -amounts of lime and of ferrochromium. This may or may not beenough in relation to the silica produced in subsequent reduction of theoxidized material with silicon, etc.

It, however, does give a composition not subject to change in the airand therefore advantageous for shipping. More lime can be physicallyadmixed before the exothermic action if it be wanted. Anotheradvantage-in roasting in the presence of lime is that oxidation may bereadily carried beyond the CraOa point with production of CrOa. Inreaction with silicon, CrOs produces considerably more heat than CrzOs.

If desired, the roasting may convert all4 the chromium to chromate. AButit is usually better to have in the roasted product a considerablepercentage of CrnOa. The reaction of CrOs with4 silicon is, as stated,more highly exothermic 'than that of .Cr203;l that is, .for equalquantities of chromium more heat is developed, the proportion of oxygenin CrOa being double that of CrzOa and the excess being, so to speak,loosely bound.

However, more slag is formed per unit of chromium as CrOa, more siliconbeing oxidized toA silica. The yield of chromium metal per unit ofsilicon used in reduction is greater from CrzOa than from Cros. It hasbeen found advantageous in some cases to carry the roasting offerrochromium so far as to give only a minor proportion of the chromiumas chromate. However in other cases the proportion may be higher. Equalproportions of chromium in the two forms give a useful product. Anydesired content of oxygen within limits can be put into the product inroastunder 1000 C. with somewhat less than 100 per cent by weight oflime with no soda ash or with less than one per cent of soda in theroasting charge converts the chromium mostly to chromate withoutcomplete oxidation and elimination of carbon from the ferrochrome metalin a reasonable length of time. AAbove 1000" the carbon is eliminatedbut the chromate breaks down to chromite. As the soda is increased thetemperature can be raised to 1200 without breaking down the chromate andthe carbon can be completely eliminated without raising the temperatureabove 1200*. Additions of soda in amounts above one per cent of thecharge or two per cent of the metal permit regulation of the temperatureto control the ratio of chromate to chromite (CrOs to CrzOs) in theoxidized product, and thus oi the oxygen content and exothermicity. Ihave found for example that addition to a roasting mix of ferrochromiumand limeof soda ash (NanCOa) in the amount of 2 to 5 per cent by Weightof the ferrochromium metal results in 50 per cent conversion of thechromium to chromate in a short time with total elimination of carbonand oxidation of metal when the temperature of roasting is held around1000 C.; the remaining chromiumA being oxidized to CrzOs appearing ascalcium chromite. With more lime or more soda the proportion of chromateis increased. A 50-50 oxidation of chromium to chromite and chromategives an oxidized material capable of a highly exothermic reactionwith-sili- 45 con alloys. Such a compound of CaO, FezOa, CrzOa and CrOacontaining a small amount of NanO from the soda ash and made fromferroehromium by roasting in air, is a highly advantageous material forexothermic production of chromium-iron metal. A fully chromated materialcontaining CaO, NazO, FezOs and CrOa in chemical combination can readilybe made. It contains less chromium but is more exothermic.

It is however possible, as stated, to oxidize high carbon ferrochromiumcompletely by roasting it in finely divided form without the presence.of lime or other oxidation promoter; roasting being in two or morestages and the material reground between the roasting stages. Thisshortens the time and fuel required. 'Oxidation of ferrochromium is 'avigorous exothermic action which starts at about 600 to 700' C., thetemperature rising automatically to 1000" C. or higher. But completeelimination of metal and of carbon requires heating of the material fora time after the exothermic action has spent itself and this time ismaterially .shortened by .the presence of a base or other oxidationpromoter with the ferrochromium. Double roasting has the advantage offacilitating grinding to a greater iineness than is usually possible forthe unroasted metal. The lime and soda are omitted with advantage in thefirst roasting and added ln subsequent regrinding and reroasting.

When 'roasting in two stages, the temperature may well be carriedconsiderably above 1000 C. in the first stage, 1350 for example, tocompletely eliminate carbon and oxidize the Cr to CraOa and afterregrinding with lime and soda a second roasting at '100 to 1000 C. putsa good percentage of CrOz in the product, a proportion which can beabove percent of the total contained chromium. `It is often advantageousin practice to roast to 100 per cent CrOa and mix the fully chromatedproduct with chromite obtained by roasting at higher temperature forshorter time. So doing, the oxygen content of the mixture is readilyladjusted a's desired for the subsequent exothermic action. t

Oxidation can be accelerated by raising the partial pressure of oxygenin wellunderstood ways, by the use of oxygen itself, or a compound.capable of releasing oxygen at the roasting temperature, such as sodiumchlorate, sodium nitrate,fsodium bichromate, chromium trioxide,manganese dioxide,l or the like. The use o1 oxygen, by raising theoxygen content of the roasting atmosphere to a point above the ordinaryconcentration of oxygen in air, naturally facilitates oxidation. A smallquantity of one of the oxygen-releasing compounds mentioned, presentalong with the lime, also shortens the roasting time, such compoundserving when so used as an effective promoter of oxidation, whilerelying upon the air as the main source of oxygen. The roasting step mayin fact be conducted, with or without lime, in the presence of a suffilcient quantity of one of the oxidizing agents or oxygen-releasingcompounds mentioned hereinabove to furnish oxygen for oxidation of mostorv all of the metal. When this is done, the oxidation becomes stronglyexothermic yand is completed in a few minutes.

However, in the ordinary practice of the present invention, air andlime, with a, little soda, are relied on. Instead of lime, strontia orbaryta may be used but their molecular weight is higher a'nd their costgreater.. Potash may be used instead of soda. The bases may be used inthe form of carbonate. Magnesia or dolomitic lime may be used. V K

The roasting can be done in a roasting or calcining furnace of suitabletype such as an open hearth or reverberatory furnace with mechanicalrabbling or 'in a rotary kiln.I Stirrlng during roasting aids oxidation.Fine grinding of the mixture in a pebble or ball mill before roasting isgood practice. l

The color of theroasted product is from black to gray to yellow,depending upon the CaO content and the CrOacontent. Complete oxidationof ferrochromium is attended with loss of magnetism and the fullyroasted material is nonmagnetic. metal and of the carbon proceedtogether, with loss of magnetic power as the metal and carbon areeliminated with formation of FezOs and without formation of FeO. Themagnetic test may be used to measure the elimination of metal, of FeOand of carbon, the roasting being stopped rwhen the material becomesnon-magnetic.`

If the synthetic chromite Vis insufiiciently high in CrrFe ratio, it maybe next beneflciated by replacement of iron with lime and preferentialreduction of iron, in the manner described in my Patent No. 2,098,176.Or the ore may be beneciated before reduction `to high carbonferrochromium. Whether or not such beneciation is practiced, theresultant chromite, which may` Ih the roasting, oxidation of the"4 becalcium ferrichromite or a' calciumchromite,

` or a chromated chromite, is then exothermically reducible with a`non-carbonaceous reducing` agent, such as ferrosilicon, ferrochromesilicon, aluminum, or the. like. 'I'his reduction results lin theproduction of an iron-chromium alloy low in carbon; e. g. chromiumsteel, low carbon ferrochromium, or even (where iron has beenselectively reduced and removed) a chromium metal of high purity. lowgrade, high iron, materials are, however, advantageous in making chromealloy steel and iron.

AAs a carbon-free reducing agent, any of the alloys of aluminum orsilicon, or magnesium, `may be used. Calcium, magnesium and aluminumsilicides are effective. Where nickel is wanted in the final alloy,nickel silicide may form va component of the reductant. Ferrochromesilicon is useful in adding chromium with the heat developed roastedferrochrome by the silicon.

I n accordance with one aspect of my invention, roasted ferrochromium ismixed with 'ferrosilicon or ferrochrome silicon, both as iine powders,in such quantity as to supply suiiicient sili- Vcon to reduce thechromium and iron oxides to metal. To make a good exothermic mixturereacting completely and quickly, it is necessary that the mixing beexceptionally complete. It is Va useful expedient, after making themixture to ball mill it for a time. The mixture is next introinexothermic reduction of` duced into a steel bath, as in the open hearthsteel furnace, in such relative quantity as to supply the desiredchromium content in the final steel product,4 This step maybe the finalstep in standard open-hearth steel manufacture; the step 'whenexothermic action is initiated and 1 completed with productionof moltenmetal which enters the steel, while the Si02 formed, aswell as any SiOzpresent in the mixture added, combines with the CaO present in thechromite to form a non-refractory slag; a slag which is .freerunning at.the steel-making temperature, and Vnot objectionable either in amountor in character. In makirig this addition to molten iron or steel, thereis no local chilling. As a matter of fact, with the usual exothermic.mixtures, there.

respondingly high. An oxygen-carrying compound such as sodium nitrate,sodium chlorate,

sodium bichromate, or the like may be added with the roastedferrochromium, and the amount of silicony added in the reducing agent ismade sumcient to supply silicon for oxidation by this added material andto give suiii'cient heat in being oxidized to melt the whole of the mix,in addition to that required forreducing the metal oxides of thechromite.

composition made by the exothermic reaction of iron oxide4 andferrosilicon. As in the method previously described,- enough lime shouldordinarily be present in the exothermic mixture to form, with theresultant silica, as well as any Iron ore may be used in` theSilico-thermic mixture and steel of a desired silica present as such inthe chromite. a slag having a lime-silica ratio of approximately 1.5:1by weight. Where somewhat higher or lower slag` ratios are desirable inthe steel-making opera.- tion, the lime and silicon may be adjustedaccordingly. i

`In using oxidized -ferrochromium as described for making chrome steelin the open hearth `furnace an advantageous procedure is to mix it inpowdered form with finely vdivided ferrochrome silicon to form asilico-thermic mixture capable of converting itself by exothermicreaction into molten chromium-iron metal and lime silicate slag; thenigniting the mixture inv an insulated furnace, allowing the reaction tocomplete itself and pouring the metal into the open hearth steel bath,with or Without the silicate slag. This procedure effects addition ofmolten low carbon ferrochromium to the open-hearth reiined steel. Theamount of chromium thus put into the steel i is that required for thedesired alloy composition.

In utilizing oxidized ferrochromium as the oxidant and ferrosilicon asthe reductant in the exothermic mixtures, all Vof the silicon can beoxidized. With a small excess of silicon over this amount, some siliconwill enter the metal.

Followlngare examples showing specific embodiments of my invention.

Example I A low grade chromite ore was reduced vwith carbon in asubmerged arc electric furnace to obtain ferrochromium metal containingsubstantially all the chromium and iron of the ore. This metal wasground with lime in a ball mill to a iineness of 100 mesh androasted forabout-one hour and a half at a maximum temperature of 1350 C. withstirring in a reverberatory furnace vto obtain complete oxidation of themetal and contained carbon. An artificial chromite, calciumferrichromite, was obtained. It contained only a trace of carbon. Thisroasted product was ground together with ferrosilicon to a iineness of100 mesh and the mixture was fed in packages to a bath of molten steelin an electric furnace. The mixture underwent a smoothv exothermicreaction delivering molten metal in the form of globules and the feedingwas stopped when sumcient chromium was added to the steel. The alloyformed contained 18.7 percent chromium and carbon. under 0.1 per cent;this carbon coming partly from that in the steel and partly from theferrosilicon.

In the above example the low grade chrome ore analysed:

The ferrochromium' obtained in smelting this ore was 42 per centchromium, 48 per cent iron, 7 per cent carbon and 3 per cent silicon.Roasting this metal, Aground and mixed with 'about equal weight of lime,produced an artificial chromite of the following composition:

, Percent CraOa 26 Fea03 29 CaO 42 S102 2.7 Carbon- Trace In utilizingthis low carbon' roasted product' (con-I taining 20 per cent availableoxygen) for making 18 per cent' chromium steel, 236 parts of thechromlte (made from 100 parts of the ferrochromium metal) ground andmixed with 61 parts of ferro,-

silicon (50 per cent Si) gave in theexothermic reaction about 120 partsof metal containing 35 per cent chromium and this metal added to 94parts of molten steel gave 213 parts stainless steel of 18.7 per centchromium content with about 180 parts slag. 'I'he slag contained limeand silica in a ratio of about 1.5 to 1 (by Weight) and 2.2 per centCrzOs. The chromium recovery from the ferrochromium metal was about 94per cent.

Ewample II To make chromium steel from roasted ferrochromium andferrosilicon alone, the 42:48 ferrochromium obtained from low gradechrome ore as in Example I was ground in a proportion of 100 parts with434 parts lime and roasted in a. gas-fired rotary kiln at 1300 C. toform563 parts of carbon-free calcium ferrichromite. This was ground with 264parts ferrosilicon (50 per cent i CaO-SiOa ratio was around 1.5:,1. Theslag contained less than 1 per cent CrzOa.

In this operation, the amount of ferrosilicon was suftlcient to reduceall the iron and chromium of the roasted chromite and to react with theNaClOa of the mixture, forming NaCl which was volatilized in thereaction. The silicon in the chrome steel product was less than 0.5 percent` and the carbon less than 0.1 per cent.

Example III A high carbon ferrochromium made by total reduction of asubstandard chrome ore, and containing 61 per cent Cr, 8 per cent C, 3per cent Si and approximately 28 per cent Fe, was ground and mixed withlime and soda ash in a proportion of 132 parts lime and 5 parts soda per100 parts metal and the mixture was roasted on an open hearth for aboutan hour at a temperature of 750 to 875 C. The roasted product was achromated chromite. substantially carbon-free, containing 28 per centCrOil 10 per cent CrzOa, 14 per cent FezOa, 44 per cent CaO, 1 per centNazO and 2 per cent SiOz. This material contains 22 per cent chromium,about 10 per cent iron and 20 per cent available oxygen as CrOa, CraOaand FezOa. The roasting converted some 69 per cent of the chromium tochromate and 31 per cent to chromite. When this oxidized material isground and mixed with 35 per cent by weight of 50 per cent ferrosilicon'and the mixture ignited, it converts itself into a low-carbon metal, 44per cent Cr and 55 per cent Fe and a lime silicate slag; one hundredparts of oxidized ferrochromium and 35 parts ferrosilicon becoming about50 parts metal and 85 parts slag. In adequate quantity the mixtureproduces free running molten metal and slag.

'I'he exothermic mixture of Example 'i'i may Abei ignited by adding itto a bath of molten iron or steel in relative quantity such as to dilutethe chromium content to that `wanted in the nished alloy. Or the mixturemay be reacted in a separate furnace and the molten metal product run`into the steel furnace. In a particular instance 4oi. the chromitemixture, the steel was tapped with a chromium content of 1.2 per cent,representing a recovery of 85 percent of the chromium `contained in thesilico-thermic mixture.

Any desired quantity of such mixture may be added without raising thesilicon content `of the steel.

I have also utilized a material composed of roasted ferrochromium andferrochrome silicon in intimate admixture for putting chromium into castiron in a 50 poundfoundry ladle, at the same time diluting the carbonand silicon vcontents of the 4cupola metal and also raising thetemperature of the metal somewhat, which facilitated the castingoperation. Increasing the strength of the castiron by some 43 per centwas an important result, making it possible to vary the castings byladle additions. The diluting action of the hot tion of the ironoxideformed in roasting ferro- `chromium as well `as otjthe iron inthesilicon alloy. Forlthis function, a roasted low vgrade ferrochromiumhigh in iron is particularly adaptironoxide and ferrosilicon added tothejjexothermic mixture. v This application is in part a continuation ofmy prior copending application Serial No. 165,- 417, iiled Sept. 23,1937.

In making the exothermic mixture of oxidized ferrochromium with anon-carbonaceous reducing agent, as described, the components of thecomposite reagent aref advantageously ground together to a iineness of100 mesh. A grain size passing through a 200 mesh screen gives adesirable intimate admixture o1' the Vcomponents reacting to produce hotchromium alloy metal.

In the accompanying drawing, I have shown for purposes of illustrationonly, and not for purposes of limitation, a flow sheet illustratingseveral l of the possible chromium recovery processes which may becarried o ut in employing the principles tof the invention. Heavy lineshave been employed to outline a complete chromium recoveryprocesscommencing with the treatment of chromite ore initially and indicatingthe production ultimately' oi desirable chromium-bearing metal products.

In dotted lines, there is illustrated an alternativev oxidizingoperation, and, in light lines, I have indicated the use of variousoxidizing agents,

other than those produced directly in the oxidiz-' suitable for use inthe production of chromium alloys which comprises oxidizingcarbon-bearing ferrochromium in iinely divided condition at atemperature between 1000 C. and 1200 C. and

`forming an oxidized product low in carbon and containing iron andchromium in oxidized forms,

. and mixing the oxidized product in the-solid state with a solid,iinely divided, non-carbonaceous reducing agent lcapable of reducing theoxidizedl forms of iron and chromium to metallic .iron and metallicchromium.

2. The method of producing a reaction mixture suitable for use in theproduction of chromium alloys which comprises oxidizing carbon-bearingferrochromium n nnely divided condition at a temperature between 1000 C.and 1200 C. and

forming an oxidized` product low in carbon and 'containing iron andchromium in oxidized forms, and mixing the oxidized product in the solidstate with a solid, finely divided, silicon-containing reducing agent.

\ 3. 4The method of producing a reaction mixture suitable for use in theproduction of chromium alloys which comprises oxidizing carbon-bearingferrochromium in iinely dividedI condition and in the presence of limeand soda ash at a temperature between 1000 C. and 1200 C. and forming an'oxidized product low in carbon and containing iron and chromium inoxidized forms, and mixing the oxidized product in the solid state witha solid, finely divided, non-carbonaceous reducingagent capable ofreducing the oxidized forms of iron and chromium to metallic ironandmetallic p l chromium.` 1 ed, further diluttion being eii'ected, ifdesired, by

4. The method or producing a reaction mixture suitable for use in theproduction of chromium alloys which comprises oxidizing carbon-bearingferrochromium in iinely :divided condition and in the presence of limeand soda ash at a temperature between 1000 C. and 1200 C. and forming anoxidized product low in carbon and containing iron .and chromium inoxidized forms, and mixing the oxidized product in the solid state witha solid, iinely divided, silicon-containing reducing agent. V A

5. The method of producing a reaction mixture suitable for use in theproduction of chromium alloys which comprises oxidizing carbon-bearingferrochromium in finely divided condition and inthepresence of lime andan amount of soda, ash equal to about 2 to 5 percent of the weight ofthe ferrochromium at a temperature between l1000 C. and 1200 C. andforming an oxidized product low in carbon and containing iron andchromium in oxidized forms, and mixing the oxidized product in the solidstate with a solid, nely divided, non-carbonaceous reducing agentcapable of reducing the oxidized forms of iron and chromium to metalliciron and metallic chromium.

l6. .The methodof producing a reaction mixture suitable for use in theproduction of chromium alloys which `comprises oxidizing carbon-bearingferrochromium in iinely divided condition and in the presence of limeand an amount oi soda ash equal to about 2 to 5 percent4 of the Weightof the ierrochromium at a temperature between 1000 C. and 1200 C. andforming an oxidized product low in carbon and containing iron andchromium in oxidized forms, and mixing the oxidized product in the solidstate with a solid, finely divided, silicon-containing reducing agent.

